Method and device for producing three-dimensional models using a binding agent system

ABSTRACT

The invention relates to a method, a device and a binding agent system for producing three-dimensional models.

FIELD OF THE INVENTION

The invention relates to a method and a device as well as a bindersystem for producing three-dimensional models.

A method for producing three-dimensional objects from computer data isdescribed in the European patent specification EP 0 431 924 B1. In thismethod, a particulate material is applied in a thin layer to a platform,and a binder material is selectively printed on the particulatematerial, using a print head. The particle area onto which the binder isprinted sticks together and solidifies under the influence of the binderand, if necessary, an additional hardener. The platform is then loweredby a distance of one layer thickness into a build cylinder and providedwith a new layer of particulate material, which is also printed asdescribed above. These steps are repeated until a certain, desiredheight of the object is reached. A three-dimensional object is therebyproduced from the printed and solidified areas.

After it is completed, this object produced from solidified particulatematerial is embedded in loose particulate material and is subsequentlyremoved therefrom. This is done, for example, using an extractor. Thisleaves the desired objects, from which powder deposits are removed, forexample by manual brushing.

Of all the layering techniques, 3D printing based on powdered materialsand the supply of liquid binder is the fastest method. This method maybe used to process different particulate materials, including naturalbiological raw materials, polymers, metals, ceramics and sands (not anexhaustive list).

For example, a solid in the particulate material may be used as thebinding system. This solid is dissolved by means of a solvent which isexpelled from the ink-jet print head. After the solvent evaporates, theparticles stick together in the desired locations.

The component may be removed from the remaining loose powder after acertain waiting period. The waiting period is generally long, since thesolvent is only slowly released from the dissolved material. Thecomponents are often weak after unpacking and can be plasticallydeformed. The volatilization of the solvent produces a certain depositbuildup on the component, which must be removed manually afterunpacking. The solvent may additionally attack the print head. Moreover,the dissolution process and subsequent resolidification causes shrinkagein the component and thus also geometric deviations.

A solvent may also be loaded with molecules or particles and then used.This may reduce shrinkage. The aggressiveness of the solvent may also bedecreased while maintaining the same component strength. However, thesolvent must be completely removed before unpacking, and the problem ofdeposit buildup occurs here as well.

Another option is to use a system that chemically results in asolidification of the printed fluid and thereby causes a binding of theparticles. The system components are kept separate in the system, ifpossible. The desired solidification reaction does not occur until theprinting process. One example of a system of this type may be a methodknown as the cold resin process. An acid-encased sand is brought intocontact with furfuryl alcohol. This results in a chemical reaction whichcauses the previously liquid components to be converted to across-linked plastic.

These systems significantly reduce the aforementioned shrinkage. Themonomers used nevertheless present a danger to the print head. Themonomers for processes of this type often have an aggressiveness that iscomparable to that of solvents. The more or less latent cross-linkedplastics pose a permanent risk to the print head, since they maysolidify at any time, due to contaminants or undesirable catalysisreactions, and thus damage the print head.

Due to their high reactivity, both systems are hazardous materialsystems which may be harmful to the environment and may be used only inindustrial environments.

Another way to further minimize the aforementioned problems is to use aninitiator system which is not present in the powder. Radiation-hardeningsystems are frequently described in the literature. They have certaindisadvantages, depending on the chemical and physical hardening system.In the case of UV-hardening systems, for example, the complete,layer-by-layer hardening is a disadvantage, since it results in a delayin the build process. Another disadvantage is that no layer bondingoccurs with excessively hard irradiation. Pure IR-hardening systems alsosuffer from this problem.

One object of the invention was therefore to provide a binder system, amethod and a device which avoids or at least reduces the disadvantagesof the prior art.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, the invention comprises a method for producingthree-dimensional (3D) molded bodies (components) by means of 3Dprinting.

In another aspect, the invention comprises a device for carrying out the3D printing method according to the invention.

In one aspect, the invention comprises a binder system which includes atleast two components.

DETAILED DESCRIPTION OF THE INVENTION

A number of terms in the invention are explained in greater detailbelow.

Within the meaning of the invention, “3D printing methods” are allmethods known from the prior art which facilitate the construction ofcomponents in three-dimensional molds and are compatible with thedescribed method components and devices. In particular, these arepowder-based methods, for example SLS (selective laser sintering).

A “binder” or “binder system” within the meaning of the invention iscomposed of two components and generally relates to a novolak or resolsystem having a solvent. All known 3D printing-compatible phenols andalcoholic and aqueous solvents may be used which are known to thoseskilled in the art and therefore do not need to be described in greaterdetail here. Special components and phenols are indicated in thefollowing description. Acids or bases are required for producing thenovolak or resol systems of the invention, it being possible to use allacids and bases known for novolaks and resols which are known to thoseskilled in the art.

The “prepolymerisates” contained in the binders of the invention mayvary depending on the application and are adapted to the other materialcomponents, such as particulate materials.

All materials known for powder-based 3D printing, in particular sands,ceramic powders, metal powders, plastics, wood particles, fibrousmaterials, celluloses and/or lactose powders, may be used as“particulate materials.” The particulate material is preferably a dry,free-flowing and a cohesive, firm powder.

“Temperature regulation” of the particulate material is understood tomean that the powder material applied to the build space is held in itstotality within a certain temperature window.

Within the meaning of the invention, a “heating” or “heat treatment”after the application of binder is the selective heating of the materialareas provided with binder for building up the component. Thetemperature is significantly increased compared to the ambienttemperature and the temperature-regulated particulate material on thebuild space.

According to the invention, the selective heating is carried out noearlier than after each second application of particulate material orafter a constant or varying number of application steps. Within themeaning of the invention, the particulate material is distributed evenlyover the build space and is smoothed off before another application ofparticulate material takes place.

Within the meaning of the invention, “selective binder application” or“selective binder system application” may take place after eachparticulate material application or irregularly, depending on therequirements of the molded body and for the purpose of optimizing theproduction of the molded body, i.e., non-linearly and not in parallelafter each particulate material application. “Selective binderapplication” or “selective binder system application” may thus be setindividually and during the course of producing the molded body.

“Finishing treatment steps” or “additional treatment steps” within themeaning of the invention are all methods known to those skilled in theart, to which the molded body obtained by the 3D printing process may besubjected, for example another heat treatment.

“Molded body” or “component” within the meaning of the invention are allthree-dimensional objects that are produced with the aid of the methodaccording to the invention and/or the device according to the inventionand which have a nondeformability.

Any known 3D printing device that contains the necessary components maybe used as the “device” for carrying out the method according to theinvention. Common components include a coater, a build space, a meansfor moving the build space or other components, a dosing device and aheating means and other components which are known to those skilled inthe art and therefore do not need to be listed in greater detail here.

“Counting means” for the particle layer application within the meaningof the invention may be a mechanical or other type of means, which issuitable for measuring the number of particle layer applications. It maybe coupled with a control unit for other components or functions and/orsoftware.

The invention, along with its preferred specific embodiments, isdescribed in greater detail below.

In particular, the invention relates to a method for producing acomponent (3D molded body), wherein (a) a particle layer is applied to abuilding platform (102) in a first step with the aid of a powder coater(101); (b) a binder (400) is selectively applied in a second step withthe aid of a binder dosing device (100); (c) the applied layer or layersis/are subjected to a heat treatment in another step with the aid of aheat source (600); (d) the building platform (102) is lowered by thethickness of one layer, or the powder coater (101) and possiblyadditional device components is/are raised by the thickness of onelayer; steps a) through d) are repeated until the component is built up,the heat treatment (c) being carried out after every second oradditional layer application step.

The heat treatment is preferably carried out at a temperature of 100° C.to 170° C., preferably 130° C. to 160° C.

The invention also relates to a binder system which is suitable for a 3Dprinting method, comprising or including at least one adhesive and asolvent and possibly other additives.

The binder system according to the invention preferably contains athermally secondarily cross-linkable prepolymerisate as the adhesive andone or multiple alcohols and/or water as the solvent. The binder systemaccording to the invention particularly preferably comprises or includesa novolak and/or resol system and a solvent and possibly otheradditives.

A binder system which includes a novolak and/or resol system, as used inthe shell molding method, has proven to be particularly suitable (e.g.,Corrodur® from Hüttenes-Albertus Chemische Werke GmbH).

All suitable solvents may be used in the binder system according to theinvention, the solvent being an alcohol, preferably ethanol and/or2-propanol, and/or an aqueous solvent, preferably water and 2-propanol.

The binder system according to the invention preferably contains otheradditives, which are selected from the group of tensides and antifoamingagents.

In one preferred specific embodiment, the binder system according to theinvention is characterized in that the solvent is alcohol-based andpreferably contains no more than 30% resin.

On one particularly preferred specific embodiment, the binder systemaccording to the invention furthermore includes up to 5% polyol,preferably glycol, propylene glycol or xylitol.

The binder system according to the invention preferably has one ormultiple of the following advantageous characteristics at roomtemperature: In the preferred specific embodiment, the viscosity isbetween 5 and 40 mPas, particularly preferably between 8 and 20 mPas; asurface tension of 20 to 40 mN/m is preferred, 25 to 35 mN/m beingparticularly preferred. In one preferred specific embodiment, the vaporpressure of the binder system is no higher than 55 hPa, particularlypreferably no higher than 40 hPa.

The method according to the invention advantageously represents achemical system comprising a solvent and a binding component, in whichit was surprisingly possible to achieve the fact that high unpacking andfinal strengths were obtained by selecting the special components. Ahigh edge sharpness without deposits is also advantageously achieved,due to the lack of a reaction part in the particulate material.According to the invention, an aggressive solvent is furthermoreavoided, and the print head is thus advantageously not subjected to thedanger of damage. This guarantees continuous machine runtimes and avoidsthe need to repair or replace machine parts. On the whole, the avoidanceof aggressive solvents, which are required in other methods according tothe prior art, has many practical advantages, such as a reduced threatto material and personnel, as well as positive economical consequences,due to continuous machine runtimes and the avoidance of damage to theprinting machines.

It has proven to be particularly advantageous if the heat treatment stepin the method according to the invention is not carried out after eachlayer application and binder application but instead is carried out onlyafter every second, preferably after every second to seventh, layerapplication. The heat treatment step is particularly advantageouslycarried out after every second, preferably after every fourth to sixth,layer application, most preferably after every fifth layer application.

The hardening operation is thus preferably carried out in layers but notwith each consecutive layer. Among other things, this results in afaster workflow and thus an accelerated production speed.

This surprisingly achieves the fact that a better binding of the layersis achieved in the component and thus a frequently occurring “flaking”of the layers is avoided.

It has surprisingly proven to be particularly advantageous if theparticulate material is temperature-regulated on the building platform.The component may be thereby produced in an even better quality,depending on the particulate materials (sands) used. It has proven to beadvantageous if the particulate material is held at a temperature thatis no less than 50° C. to 70° C., preferably no less than 55° C. to 65°C., most preferably no less than 60° C.

The component produced by means of the method according to the inventionmay be subjected to additional known work steps, such as an additionalheat treatment step, preferably at a temperature of 150° C. to 200° C.This heat treatment step preferably takes place after unpacking.

In the method according to the invention, the binder described above orthe binder system described above is preferably used. The binder or thebinder system is preferably selectively applied with the aid of a binderdosing device and selectively solidifies the particulate material.

In principle, the method has the advantage that nearly all materialsknown in 3D printing methods may be used. The particulate material ispreferably selected from the group comprising sands, ceramic powders,metal powders, plastics, wood particles, fibrous materials, cellulosesand/or lactose powders. The particulate material is particularlypreferably a dry, free-flowing powder, a cohesive, firm powder or aliquid-based dispersion.

The practical and economical advantages of the method according to theinvention are excellent. The method is not bound to specific particulatematerials (sands) and may be used practically universally, in contrastto known methods from the prior art. One example is the furan resinmethod, in which no alkaline sands may be used.

Compared to known methods, the particulate material (the sand) also doesnot have to be pretreated or premixed. The residual sand, which does notform the component, may be reused and recycled in the method without agreat deal of effort. An admixture process, which is otherwisenecessary, or complex cleaning steps are dispensed with. The materialonly has to be sieved. Material costs are thereby saved, while handlingeffort is reduced and labor costs cut. This has a positive effect on thecomponent costs.

It is also possible to process the components without an additional heattreatment step.

Depending on the geometry, the binder content may furthermore be variedglobally as well as selectively by means of the print resolution as wellas selectively within the geometry. This is also not possible with knownprior-art methods, since possible mixing ratios of the reactants may nolonger be in the optimum range during printing, which results in qualityproblems. One example of this is the furanic system, since the acidcauses problems in the sand unless the correct amount of reactant ispresent.

In another aspect, the invention is a device for 3D printing, whichincludes (a) a powder coater (101), a building platform (102), at leastone binder dosing device (100), at least one heat source (600),preferably an IR emitter (604), preferably a lifting device (605) forlowering and raising the building platform (102) or the powder coater(101) and possibly other components, such as the binder dosing device(100) and the heat source (600), preferably an extraction device (606)and a counting means (607) for counting the particle powder layersapplied. The device preferably also has a contact heater (602) or a hotair device (608).

In particular, the binder dosing device (dosing device) may comprise,for example, an ink-jet print head, which selectively doses the binderonto the build space in individually dispensable droplets. The binderdosing device may also comprise a filament dispensing system, the binderbeing selectively dosed onto the build space in the form of a thin,switchable filament.

The device according to the invention and the method according to theinvention may be used in all 3D printing methods, preferably inpowder-based 3D printing processes.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic representation of the components of apowder-based 3D printer in a sectional isometric view;

FIG. 2 shows a sequence of a conventional 3D printing process with theuse of a layered radiation hardening technique;

FIG. 3 shows a sequence of a building process, using a radiationhardening technique, which does not take place with each layer;

FIG. 4 shows a diagram of the binder diffusion process;

FIG. 5 shows combined energy supplies via an IR emitter and convectiveheating of the powder;

FIG. 6 shows a chemical reaction equation for solidifying a novolaksystem, using a formaldehyde source with the introduction of heat;

FIG. 7 shows details of the reaction.

Additional details, preferred specific embodiments and advantages of theinvention are discussed below.

According to the invention, a system is essentially used for buildingmodels in layers with the aid of ink-jet printing. The sequenceaccording to the prior art is as follows: a layer of powder is appliedto a building platform and leveled. A fluid is then printed onto thelayer according to the layer data of the 3D model. The printed areaschange one or multiple properties (strength, water solubility, etc.).The material usually solidifies due to a binding component in the liquidbinder (e.g., an adhesive). The building platform is subsequentlylowered, and the process begins all over again.

A very low viscosity is generally needed for the ability to print aliquid with the aid of an ink-jet print head. To do this, the liquidbinding components (binder) usually have to be diluted. Solvents aregenerally used for this purpose. If each layer is heated, e.g., with theaid of radiation, the surface temperature of the layer to be printed isalso very high. As a result, the solvent very quickly evaporates. Thebinder cannot penetrate deeply enough into the layer and thereby bindthe layers together. The structural body will flake apart. In manycases, the strategy of heating less does not work, since thesolidification sets in only above the evaporation temperature of thesolvent. A positive process window cannot be found in known methods.

By means of the method according to the invention and the deviceaccording to the invention, a stable, firm-edged and well definedstructural body may be advantageously produced, for example if hardeningtakes place only after every fifth layer application. The solidificationis thus completed only after adequate diffusion of the binding component(binder/binder system). The evaporation of the solvent of the fifthlayer surprisingly does not impair the structural body.

In test series, it was possible to document that the number of exposurescan increase, while no relationship between the layering time, liquidsupply and IR radiation power resulting in satisfactory structuralbodies was found when exposing every layer.

The system according to the invention draws heavily on powder-based 3Dprinting. The mechanical engineering of the device according to theinvention has been expanded according to the requirements of the methodaccording to the invention.

The device according to the invention includes a power coater.Particulate material is applied thereby to a building platform andsmoothed (FIG. 2(a)). The applied particulate material may comprise awide range of materials. For example, sands, ceramic powders, metalpowders, plastic, wood particles, fibrous materials, celluloses, lactosepowders, etc. may be used. The flow characteristics of these materialsmay vary enormously. Different coater techniques permit layering fromdry, free-flowing powders and cohesive, firm powders to liquid-baseddispersions. The height of powder layers is determined by the buildingplatform. It is lowered after one layer has been applied. During thenext coating operation, the resulting volume is filled and the excesssmoothed. The result is a nearly perfectly parallel and smooth layer ofa defined height.

After a coating process, a liquid is printed on the layer with the aidof an ink jet print head (FIG. 2(b)). The print image corresponds to thesection of the component at the present build height of the device. Thefluid strikes and slowly diffuses into the particulate material.

After the binder, preferably the binder system according to theinvention, is printed, the layer is solidified using the methodaccording to the invention (FIG. 2(c)). For this purpose, an IR emittermay be passed over the build space. This emitter may be coupled with theaxis of the coating system. The solvent evaporates during heating. Inthe case of liquids that present a fire hazard, the evaporating materialis extracted immediately.

The controller of a machine according to the invention may count thelayers and change the sequence only after, for example, every secondlayer and trigger a solidification run (FIG. 3). Likewise, however, theenergy supply may be estimated based on measured data, and the frequencyof the solidification runs may be adapted. Examples includesolidification runs after three, four, five or six layers of theparticulate material and preferably the binder. The printed liquidquantities, which fluctuate depending on the layer image, areessentially an interference variable of a control of this type. Insteadof control based on sensor data, the information may also be linkedwithin the controller.

FIG. 4 shows an example of a droplet which penetrates the powder(particulate material). After steps (a) through (d), the penetration isdeep enough to bind the layers. The diffusion quickly slows down, sincethe droplet has disappeared as a reservoir. If printing takes place onan excessively preheated layer, the solvent abruptly boils, and thebinder becomes highly viscous. As a result, it remains in the stateshown in FIG. 4(b). It therefore does not form a layer bond.

In addition to IR irradiation, the powder may also be preheated. Contactheaters, hot air or IR emitters are suitable for this purpose. Thispreheating makes it possible to effectively control the IRsolidification process at low lamp powers and to achieve high processspeeds.

After the solidification step, the building platform is lowered by thethickness of one layer (FIG. 2(d)). The complete component is created byrepeating the aforementioned steps.

An exemplary representation of a chemical system which may be usedaccording to the invention, based on a formaldehyde-hardenable novolaksystem, is as follows: novolaks are known from their use as a shell ofsand having the designation Croning resin. Finished solutions of suchresins and hardening additives, which are used for coating sand, may bepurchased, for example, from Hüttenes-Albertus Chemische Werke GmbH. Inpreliminary tests, alcohol-based solutions have proven to be easilyprocessed with the aid of ink-jet printing systems, due to theirviscosity as well as their compatibility with the novolak system, theresin content preferably being less than 30%. Other additives, such astensides and antifoaming agents, may be added to optimize printability,and up to 5% polyols, such as glycol, propylene glycol or xylitol, maybe added to finely adjust the viscosity. The thermal hardening of thenovolak after the selective introduction of the liquid into theparticulate material takes place by the breakdown, e.g., of urotropineas the hardening additive in ammonia and formaldehyde and ultimately bya reaction with formaldehyde with the novolak according to the reactionequation in FIG. 7. The starting temperature of the hardening additivemust be well removed from room temperature so that no undesirablereactions begin. The print solution set in this manner is thenselectively introduced in layers into the particulate material with theaid of a print head, the particulate material having a temperature ofpreferably at least 60° C. during the entire build process. Theevaporation speed of the excess solvent is increased, due to thetemperature of the sand, and is continuously removed in the continuousextraction flow. The actual hardening process takes place by furthersupply of heat to the printed particulate material, preferably with theaid of an IR lamp, the temperature temporarily rising above 160° C. Theexposure operation is preferably repeated every five layers, the buildprocess always terminating with an exposure operation. During anexposure operation, a significant change in color of the printed areas,from ocher hues to brown, indicates the polymerization. Due to theformaldehyde and ammonia released at high temperature as a catalyst, theexisting prepolymerisate is cross-linked in another condensationreaction to form a duroplast. After the final exposure step, thecomponents are preferably left in the unprinted particulate material foranother hour. To further increase the strength, the unpacked componentsmay be preferably stored in the oven for another hour at a temperaturebetween 150° C. and 200° C. The system is particularly characterized inthat the components may be effortlessly removed from the unprinted sandand have a very high edge sharpness.

LIST OF REFERENCE NUMERALS

100 Binder dosing device (dosing device)

101 Powder coater

102 Building platform

103 Component (3D molded part)

104 Build space boundary

107 Powder layers

200 Solidifying unit

400 Binder

401 Powder particles

500 Heat effect

502 Storage device

503 Extraction system

600 Heat source

601 Lifting device

602 Contact heater

603 Hot air device

604 IR emitter

605 Lifting device

606 Extraction device

607 Counting means

608 Hot air device

What is claimed is:
 1. A method for producing a component comprising thesteps of: (a) applying a particle layer including a particulate materialto a building platform in a first step with the aid of a powder coater;(b) applying a binder system in a second step with the aid of a dosingdevice; (c) subjecting the applied layer or layers to a heat treatmentin another step with the aid of a heat source; and (d) the buildingplatform is lowered by the thickness of one layer, or the powder coaterand possibly additional device components are raised by the thickness ofone layer; steps a) through d) are repeated until the component is builtup wherein the binder system includes i) a novolak and/or a resol; andii) a solvent; wherein the dosing device includes a print head, and theapplied particulate material has a temperature of at least 60° C. duringthe entire build process.
 2. A method according to claim 1, wherein theheat treatment is carried out at a temperature of 100° C.
 3. A methodaccording to claim 1, wherein the particulate material is selected fromthe group consisting of sands, ceramic powders, metal powers, plastics,wood particles, fibrous materials, celluloses and lactose powders. 4.(canceled)
 5. (canceled)
 6. (canceled)
 7. (canceled)
 8. The method ofclaim 1, wherein the solvent has a viscosity of 5 to 40 mPas.
 9. Themethod of claim 8, wherein the solvent has a vapor pressure at roomtemperature of 55 hPa or less.
 10. The method of claim 9, wherein thesolvent has a surface tension of 20 to 40 mN/m.
 11. The method of claim1, wherein the particulate material is preheated.
 12. The method ofclaim 1, wherein the binder is applied as droplets, and penetrates theparticulate material deep enough to bind the layers.
 13. The method ofclaim 1, wherein the particulate material includes a sand.
 14. Themethod of claim 13, wherein the penetration of the binder into theparticulate material is stopped by evaporation of the solvent.
 15. Themethod of claim 14, wherein the heat treatment polymerizes and/orcross-links the binder.
 16. The method of claim 13, wherein the resincontent of the binder system is less than 30 percent.
 17. The method ofclaim 13, wherein the solvent has a vapor pressure at room temperatureof 55 hPa or less.
 18. The method of claim 17, wherein the solvent has aviscosity of 5 to 40 mPas and/or the solvent has a surface tension of 20to 40 mN/m.
 19. The method of claim 16, wherein boiling of the solventis avoided so that the binder system penetrates deep enough to bind thelayers.
 20. The method of claim 19, wherein the resin system is appliedwith a print head.
 21. The method of claim 20, wherein the binder systemincludes the novolak.
 22. The method of claim 21, wherein the solventincludes an alcohol.
 23. The method of claim 22, wherein the solventincludes ethanol, propanol-2, or water.
 24. The method of claim of claim20, wherein the binder system includes the resol.